When your professor requires you to use "print" sources in a research paper, they usually do not mean that you have to access the source in hard copy.

Occasionally, there is a situation where it is important you use the print format -- if you have any doubt at all about this, ask your professor!

This requirement usually means that your professor wants you to use sources that have been published in some traditional format:  books, magazines, newspapers, newsletters, academic journals, trade periodicals, press releases, wire services, government reports or whitepapers, etc. 

The key is knowing where the source was published -- find out what you can about the publication or the publisher.

There are lots of resources that are published in those traditional formats and are also available online.  These are usually acceptable sources.  Once again, if you have any doubt - ask your instructor!